Crystallizaiton of 2,6-dimethylnaphthalene

ABSTRACT

A method for recovering crystalline 2,6-dimethylnaphthalene comprising crystallizing in a scraped-wall crystallizer apparatus at crystallization temperature T, a mixture of low melting components, LM, having melting points of 70° F. and below, and high melting components (HM), including 2,6-dimethylnaphthalene, having melting points above 70° F., such that: ##EQU1## where HM is the total weight percent of high melting components, including 2,6-dimethylnaphthalene, in the mixture, and LM is the total weight percent of low melting components in the mixture, and where T is the temperature of the crystallization in degrees Fahrenheit, and where A is at least 1.0.

FIELD OF THE INVENTION

This invention relates to an improved method for recovering2,6-dimethylnaphthalene by crystallization from a mixture containing2,6-dimethylnaphthalene and organic components formed during thepreparation or isolation of 2,6-dimethylnaphthalene. More particularly,this invention relates to a method for recovering2,6-dimethylnaphthalene from a mixture containing dimethylnaphthalenes,wherein the composition of the mixture is adjusted to provide forefficient crystallization of the 2,6-dimethylnaphthalene.

BACKGROUND OF THE INVENTION

2,6-Dimethylnaphthalene is a preferred feedstock for preparing2,6-naphthalenedicarboxylic acid. 2,6-Naphthalenedicarboxylic acid is auseful monomer for the preparation of high performance polymericmaterials. For example, 2,6-naphthalenedicarboxylic acid, or its methylester, can be reacted with ethylene glycol to preparepoly(ethylene-2,6-naphthalate) (PEN). Fibers and film manufactured fromPEN have improved strength and superior thermal properties relative toother polyester materials. Films made from PEN demonstrate, for example,superior resistance to gas diffusion and particularly to the diffusionof carbon dioxide, oxygen and water vapor. Because of its exceptionalproperties, PEN is especially suitable for applications such as food andbeverage containers, particularly for so-called "hot-fill" food andbeverage containers, tire cord, magnetic recording tape and electroniccomponents.

Although 2,6-naphthalenedicarboxylic acid can be prepared by a number ofprocesses, perhaps the most preferred because of cost and efficiency, isthe liquid phase oxidation of 2,6-dimethylnaphthalene. A suitable methodfor oxidizing 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylicacid is described, for example, U.S. Pat. No. 5,183,933 to Harper et al.Although feedstocks other than 2,6-dimethylnaphthalene, for example2,6-diethylnaphthalene or 2,6-diisopropylnaphthalene, can be oxidized to2,6-naphthalenedicarboxylic acid, 2,6-dimethylnaphthalene is preferredbecause it is lower in molecular weight compared to 2,6-diethyl- or2,6-diisopropylnaphthalene and, therefore, less 2,6-dimethylnaphthalene(by weight) is required to prepare a specified weight amount of2,6-naphthalenedicarboxylic acid.

While 2,6-dimethylnaphthalene is present in certain refinery streams,for large scale use it is preferable to manufacture2,6-dimethylnaphthalene starting with simple, readily available andinexpensive starting materials. One such process for manufacturing2,6-dimethylnaphthalene is disclosed in Lillwitz et al., U.S. Pat. No.5,198,594. The process for preparing 2,6-dimethylnaphthalene disclosedtherein is called the "Alkenylation Process," which comprises reactingo-xylene with butadiene in the presence of a zero-valent alkali metal toform orthotolylpentene (OTP). The alkali metal-promoted reaction of analkylaromatic with a conjugated diene such as butadiene to form anolefinically substituted aromatic is referred to as an alkenylationreaction. The OTP is subsequently cyclized to form 1,5-dimethyltetralin(1,5-DMT), the 1,5-DMT is dehydrogenated to 1,5-dimethylnaphthalene(1,5-DMN) and the 1,5-DMN is isomerized to the desired2,6-dimethylnaphthalene (2,6-DMN). The overall process is summarized inequations (1) through (4) below. ##STR1##

In the alkenylation process for preparing 2,6-dimethylnaphthalene, theisomerization of 1,5-dimethylnaphthalene typically produces not only thedesired 2,6-dimethylnaphthalene, but also a mixture of other hydrocarboncomponents such as 1,6- and 1,7-dimethylnaphthalenes and 1- and2-monomethylnaphthalenes and various trimethylnaphthalenes.Additionally, other processes for preparing 2,6-dimethylnaphthalene andmethods for isolating 2,6-dimethylnaphthalene from refinery streamsgenerally require recovering the 2,6-dimethylnaphthalene from a mixturecontaining other hydrocarbons. In order to have a cost-effective processfor preparing 2,6-dimethylnaphthalene, it is necessary to efficientlyisolate the 2,6-dimethylnaphthalene from the mixture of hydrocarbonsproduced by the isomerization reaction, the hydrocarbon mixturesproduced by other processes for preparing 2,6-dimethylnaphthalene, ormixtures produced during the processes for isolating2,6-dimethylnaphthalene from refinery streams. The art, therefore, needsa simple and cost effective method for recovering valuable2,6-dimethylnaphthalene from a mixture containing2,6-dimethylnaphthalene and other hydrocarbon components. The presentinvention provides such a method.

Methods for isolating 2,6-dimethylnaphthalene from mixtures ofhydrocarbons are known. For example, U.S. Pat. No. 3,806,552 to Oka etal., discloses that the separation of 2,6-dimethylnaphthalene from theisomerization reaction product can be easily carried out by cooling theisomerization reaction product to a proper temperature and separatingthe precipitated crystals, or by adding a suitable solvent to theisomerization reaction product, cooling the solution and separating theprecipitated crystals. A number of suitable solvents are disclosedtherein. U.S. Pat. No. 3,775,496 to Thompson et al., discloses thatselective crystallization has been used to separate DMN(dimethylnaphthalenes) from each other, citing U.S. Pat. Nos. 3,485,885;3,541,175; 3,590,091 and 3,594,436. U.S. Pat. No. 3,541,175 discloses aprocess for isolating 2,6-dimethylnaphthalene, and it discloses thatcrystallization may be carried out in a scraped cooling crystallizer.British Patent 1,345,479 discloses that 2,6-dimethylnaphthalene can beisolated by crystallization. As an example, 2,6-DMN is crystallized froma mixture containing 35.3 mol. % to 2,6-DMN, 36.8% 1,6-DMN, 8% 1,5-DMN,7.7% 1,7-DMN and 4.2% low-boiling point and high-boiling pointby-products.

SUMMARY OF THE INVENTION

A method for preparing crystalline 2,6-dimethylnaphthalene comprisingcrystallizing in a scraped-wall crystallizer apparatus atcrystallization temperature T, a mixture of low melting components, LM,having melting points of 70° F. and below, and high melting components,HM, including 2,6-dimethylnaphthalene, having melting points above 70°F., such that: ##EQU2## where HM is the total weight percent of highmelting components, including 2,6-dimethylnaphthalene, in the mixture,and LM is the total weight percent of low melting components in themixture, and where T is the temperature of the crystallization indegrees Fahrenheit, and where A is a value no more than about 1.0.

This invention is also a method for preparing crystalline2,6-dimethylnaphthalene comprising maintaining, at a temperaturesufficient to cause the crystallization of 2,6-dimethylnaphthalene, amixture comprising 2,6-dimethylnaphthalene, at least one of 1,6- or1,7-dimethylnaphthalene, and at least about 5 weight percentlight-boiling components produced during the preparation or isolation of2,6-dimethylnaphthalene.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows in graphical form preferred parameters for operating themethod of this invention.

FIG. 2 is a flow diagram showing a preferred embodiment for operatingthe method of this invention.

DETAILED DESCRIPTION OF THE INVENTION

We discovered that 2,6-dimethylnaphthalene can be effectivelycrystallized in a scraped-wall crystallization apparatus provided theweight ratio of high melting components (HM), which includes2,6-dimethylnaphthalene, to low melting components (LM) in the mixturebeing crystallized is as follows: ##EQU3## where T is the temperature,in degrees Fahrenheit, used for the crystallization and A is no morethan about 1.0. We have determined that when the ratio of HM/LM isoutside of this range, the scraped-wall crystallizer cannot effectivelycrystallize the 2,6-dimethylnaphthalene. Specifically, the mixture beingcrystallized forms a solid mass that clings to the scraping device inthe crystallizer and the crystallizer becomes inoperable. In contrast,when the crystallization operation is conducted within the operablerange of equation (5), the product exiting the scraped-wall crystallizeris in the form of a slurry of crystalline 2,6-dimethylnaphthalene in thecrystallization mother liquor. Additionally, we have determined that theweight ratio of high melting components to low melting components in themixture subjected to crystallization can be adjusted to the desiredcomposition by utilizing the light boiling components (lights) that areproduced during the preparation or isolation of 2,6-dimethylnaphthalene.These lights, for example, can comprise the mixture of components havinga lower boiling point than the dimethylnaphthalenes, and whichcomponents are produced during the process steps for preparing2,6-dimethylnaphthalene using the alkenylation process. Instead ofremoving these lights, we have determined that the amount of lightspresent during the crystallization of the 2,6-dimethylnaphthalene can beadjusted by, for example, fractionation prior to crystallization. Thecomposition produced thereby provides for the efficient crystallizationof 2,6-dimethylnaphthalene, particularly when a scraped-wallcrystallizer apparatus is used to conduct the crystallization. Inequation (5) above, T is preferably in the range of 50° to about 150°F., more preferably 60° to about 140° F.

The method of this invention is advantageously used to isolate2,6-dimethylnaphthalene produced by the alkenylation process. Asdescribed hereinabove and summarized in Equations (1) to (4)hereinabove, the alkenylation process comprises four basic chemicalreaction steps. In the first step, the alkenylation reaction, o-xyleneis reacted with butadiene to produce an orthotolylpentene (OTP). The OTPformed is actually a mixture of 5-OTP-1 and cis- and trans-5-OTP-2. Asuitable method for preparing orthotolylpentene is disclosed in Lillwitzet al., U.S. Pat. No. 5,198,594; other methods are disclosed in U.S.Pat. Nos. 3,766,288 and 3,953,535 to Shima et al. The specification ofthese three patents are hereby incorporated by reference. In thisalkenylation reaction, o-xylene in the liquid phase is reacted with1,3-butadiene in the presence of a catalytic amount of an alkali metal.Typically, an excess molar amount of o-xylene is used in order to reducethe amount of high molecular weight by-products that are formedresulting from the reaction of more than one molecule of butadiene permolecule of o-xylene. For example, 1,3-butadiene can be reacted withabout 1.1 to about 20 moles of o-xylene per mole of 1,3-butadiene, inthe liquid phase, at a temperature of about 200° F. to about 320° F. andat a pressure of about 1 atmosphere to about 3 atmospheres to form aproduct mixture comprising orthotolylpentene. The reaction is suitablycatalyzed by an alkali metal such as potassium or a mixture of potassiumand sodium, preferably NaK. The amount of alkali metal is about 1 toabout 10,000 parts by weight per million parts by weight of o-xylene.After the alkenylation reaction, the reactive alkali metal can bequenched with, for example, water or an alcohol, and the excess o-xyleneis removed to produce a concentrate containing the desiredorthotolylpentene. The concentrate can be fractionated to removematerials having boiling points higher and/or lower thanorthotolylpentene to form a purified form of orthotolylpentene. However,the concentrate containing components having a lower and higher boilingpoint than orthotolylpentene can also be used directly in thecyclization step.

In the cyclization step of the alkenylation process, theorthotolylpentene is cyclized using a catalyst to form1,5-dimethyltetralin. Methods for cyclizing orthotolylpentene, as wellas suitable catalysts, are disclosed in U.S. Pat. Nos. 5,034,561;5,030,781 and 5,073,670, to Sikkenga et al., the specifications of whichare hereby incorporated by reference. Other processes for cyclizingorthotolylpentene to dimethyltetralin are disclosed, for example, inU.S. Pat. Nos. 3,775,498; 3,775,496 and 3,840,609, the specifications ofwhich are hereby incorporated by reference. The cyclization reaction canbe conducted using either a gaseous or liquid phase reaction. Apreferred method for cyclizing orthotolylpentene to 1,5-dimethyltetralinuses an acidic molecular sieve catalyst such as a low-acidity,ultrastable crystalline aluminosilicate zeolite Y in the hydrogen formhaving a silica-to-alumina bulk molar ratio of at least about 12, a unitcell size no greater than 24.3 Angstroms, a sodium content of no morethan about 0.4 weight percent sodium or, in terms of sodium oxide, asodium oxide -to-alumina bulk molar ratio in the range of about 0.001:1to about 1:1. The cyclization reaction comprises maintaining theorthotolylpentene in the liquid phase at a temperature of about 120° C.to about 350° C., at a pressure, for example, of about 0.05 to about 0.5atmospheres, to form a reaction mixture comprising the1,5-dimethyltetralin. The weight ratio of orthotolylpentene to themolecular sieve cyclization catalyst is suitably about 1000:1 to about10:1. The mixture produced by the cyclization reaction can be useddirectly for the next reaction in the alkenylation process;alternatively, the reaction product mixture from the cyclizationreaction can be fractionated to remove light and/or heavy components andthereby prepare a concentrate of 1,5-dimethyltetralin.

In the next step of the alkenylation process, the mixture produced bythe cyclization reaction, or the concentrate prepared by fractionatingthe product from the cyclization reaction, is dehydrogenated to prepare1,5-dimethylnaphthalene. This dehydrogenation can be accomplished usingeither a gas phase reaction or a liquid phase reaction. Suitable methodsand catalysts for conducting such a dehydrogenation reaction aredisclosed in U.S. Pat. Nos. 5,118,892; 5,189,234; 3,775,498 and3,781,375, the specifications of which are hereby incorporated byreference. A preferred method comprises contacting the1,5-dimethyltetralin in the gas phase with a suitable dehydrogenationcatalyst such as a catalyst comprising alumina, about 0.05 to about 5.0weight percent platinum or palladium, no more than about 0.14 weightpercent halide, and about 0.10 to about 2.0 weight percent alkali metal,all weight percents based on the weight of the catalyst, at atemperature of about 600° to about 900° F., a pressure of about 0.01atmosphere to about 25 atmospheres, and at a weight hourly spacevelocity of about 0.1 hr⁻¹ to about 20 hr⁻¹. The mixture produced by thedehydrogenation reaction can be used directly in the isomerizationreaction; alternatively, it can be fractionated to remove lower boilingcomponents and/or higher boiling components, for example, those formedduring the dehydrogenation reaction. The product from thedehydrogenation reaction, either in fractionated or unfractionated form,is isomerized over a suitable catalyst to form a mixture ofdimethylnaphthalenes, including the desired 2,6-dimethylnaphthalene.Suitable methods for isomerizing 1,5-dimethylnaphthalene are disclosed,for example, in U.S. Pat. Nos. 4,962,260; 4,950,825; 3,775,498;3,781,375; 3,855,328 and 3,957,896, the specifications of which arehereby incorporated by reference. A suitable method for isomerizing1,5-dimethylnaphthalene comprises contacting 1,5-dimethylnaphthalene inthe liquid phase at a temperature in the range of about 200° C. to about420° C. with an acidic isomerization catalyst. For example, the catalystcan comprise either an acidic ultrastable crystalline y-zeolite having asilica-to-alumina molar ratio of from about 4:1 to about 10:1, havingpore windows provided by twelve-membered rings containing oxygen and aunit cell size of from about 24.2 to about 24.7 angstroms, orbeta-zeolite.

The product from this isomerization reaction contains2,6-dimethylnaphthalene. Preferably, the product from the isomerizationcontains at least about 20, more preferably at least about 35 weightpercent 2,6-dimethylnaphthalene. In addition, depending on theconditions used for the isomerization, and whether or not priorfractionation steps were used to remove high boiling and low boilingcomponents from the product mixtures produced during the precedingalkenylation, cyclization, dehydrogenation and isomerization steps, theproduct mixture produced by the isomerization reaction can contain inaddition to the dimethylnaphthalenes, about 1 to about 20 weight percentof light, low boiling components (lights), about 0.5 to about 5 weightpercent of various trimethylnaphthalenes and about 0.5 to about 5 weightpercent of the heavy, high boiling components (heavies). By light, lowboiling, we mean that the boiling point of the component is lower thanthe boiling point of 2-methylnaphthalene. Thus, the light, low boilingcomponents (lights) have a boiling point lower than about 500° F.,preferably lower than about 470° F., at atmospheric pressure. By heavy,high boiling, we mean having a boiling point greater than the boilingpoint of any of the dimethylnaphthalenes, more preferably greater thanthe boiling point of the trimethylnaphthalenes. Thus, the heavy, highboiling components (heavies) have a boiling point greater than about520° F. at atmospheric pressure. In addition to the above, the productproduced by the isomerization reaction can contain various otherdimethylnaphthalenes in addition to the desired 2,6-dimethylnaphthalene.Such dimethylnaphthalenes include one or more of 1,6- and1,7-dimethylnaphthalene, preferably at least 1,6-dimethylnaphthalene,and typically about 40 to about 80 weight percent of such otherdimethylnaphthalenes.

As described hereinabove, instead of removing the light, low-boilingcomponents prior to crystallizing the 2,6-dimethylnaphthalene, they canbe included, in variable amounts, in the mixture fed to the crystallizerin order to improve the crystallization of the 2,6-dimethylnaphthalene.The amount of lights included in the mixture fed to the crystallizerapparatus is an amount that provides for the efficient crystallizationof 2,6-dimethylnaphthalene, suitably at least about 5 weight percent ofthe total mixture crystallized, preferably at least about 7 weightpercent, and most preferably at least about 10 weight percent of thetotal mixture subjected to crystallization. Preferably no more thanabout 25 weight percent of the mixture subjected to crystallization islights.

In addition to the amount of lights present in the mixture crystallized,the relative amounts of the other components in the mixture affects thecrystallization of the 2,6-dimethylnaphthalene. As stated hereinabove,the weight ratio of high melting components, HM, to low meltingcomponents, LM, must be: ##EQU4## wherein the high melting componentshave melting points above about 70° F. and can include, for example,2,6-dimethylnaphthalene, 1,5-dimethylnaphthalene,2,7-dimethylnaphthalene and 2-methylnaphthalene (2-MN). The low meltingcomponents include, for example, 1,6-dimethylnaphthalene,1,7-dimethylnaphthalene, 1-methynaphthalene and other light boilingcomponents, the trimethylnaphthalene fraction and the heavy boilingcomponents. The value of A is no greater than 1.0, more preferably nogreater than 0.9. Most preferably, A is about 1.0 to 0.9. Thus, we havefound that a mixture containing the composition X shown below could notbe crystallized in a scraped-wall crystallizer to isolate2,6-dimethylnaphthalene at a crystallization temperature of 70° F.whereas, the composition Y shown below was successfully crystallized at85° F. to yield 2,6-dimethylnaphthalene in high yield and purity using ascraped-wall crystallizer apparatus to conduct the crystallization. Asshown below, the feed X has a HM/LM ratio of 1.6 and at acrystallization temperature of 70° F., equation (5) is not satisfiedwhere A is 1.0. Composition Y, however, has a HM/LM value of 1.2 and ata crystallization temperature 85° F., equation (5) is satisfied where Ais 1.0.

    ______________________________________                    CRYSTALLIZER FEED                    COMPOSITION (WT. %)    COMPONENT         X         Y       Z    ______________________________________    Lights.sup.a      4.0       0.7     7.2    2-MN              0.9       0.6     3.1    1-MN              0.3       0.5     1.1    1,5-DMN           9.2       13.3    7.5    1,6-DMN           33.0      41.4    27.6    2,6-DMN           41.4      39.0    43.5    1,7-DMN           0.8       1.8     1.4    2,7-DMN           9.3       2.4     6.1    Heavies           1.1       0.4     2.5    Cryst. Temp. °F.                      70        85      70    HM/LM             1.6       1.2     1.5    0.03 (T-50) + 1.0 1.6       2.0     1.6    HM/LM < 0.03 (T-50) + 1.0                      No        Yes     Yes    CRYSTALLIZED    2,6-DMN (Yield, %/Purity, %)                      inoperable                                75.6/99 77.8/97.5    ______________________________________     .sup.a Other than 1MN

Thus, it is not only the quantity of the light boiling components in themixture that provides for efficient crystallization, but the weightratio of high melting components to low melting components and thecrystallization temperature. At times, the ability to adjust the ratioof high melting components to low melting components is not feasibleunless the adjustment is made by altering the level of light boilingcomponents, i.e., a type of low melting component, in the feed mixtureto the crystallizer. For example, mixture Z in the table above, whichmixture contained more lights than mixture X, was effectivelycrystallized in a scraped-wall crystallizer apparatus providing2,6-dimethylnaphthalene in good purity and high yield. Using the lightsto assist in the crystallization of 2,6-dimethylnaphthalene isadvantageous because it does not introduce extraneous components intothe process stream, which would occur if a solvent were used. Aftercrystallization, and after the desired crystalline2,6-dimethylnaphthalene is separated from the mother liquor, the motherliquor, which contains the light and heavy components, can befractionated into its various components. One or more of such fractionscan be recycled to the isomerization or crystallization step if desired,or one or more can be purged from the process and used, for example, asfuel.

In addition to using lights to adjust the HM/LM ratio, the1,5-dimethyltetralin (1,5-DMT) produced in the second step of thealkenylation process can also be used. This component, which has amelting point of less than 70° F., is also highly suitable because itdoes not add extraneous materials to the process. When used, the amountof 1,5-DMT in the mixture crystallized is suitably at least about 2 wt.% of the mixture crystallized, preferably at least about 5 wt. % andmore preferably at least about 7 wt. %.

The lights (i.e., light, low boiling components) useful in the method ofthis invention comprise a complex mixture of compounds and where suchmixture is produced during the manufacture or isolation of2,6-dimethylnaphthalene. Preferably, the lights comprise a mixture oforganic compounds which mixture has a boiling point lower than theboiling point of 2-methylnaphthalene, and more preferably where they areobtained by fractionating the mixture produced in the alkenylationprocess subsequent to the isomerization step where1,5-dimethylnaphthalene is isomerized to 2,6-dimethynaphthalene. Thelights mixture has a melting point below 70° F., and thus is alow-melting component. These lights typically contain o-xylene,1-methyl-2-pentylbenzene, 5-orthotolylpentene-1, 5-orthotolyl-pentene-2,1,5-dimethyltetralin, 1,6-dimethyltetralin, other dimethyltetralinisomers, and 1-methylnaphthalene.

As described hereinabove, the method of this invention is also usefulfor isolating, by crystallization, 2,6-dimethylnaphthalene from othersources or produced by other synthetic routes. For example, the methodof this invention can be used to crystallize 2,6-dimethylnaphthaleneisolated from fractions obtained from the catalytic or thermal crackingof petroleum, such as the dimethylnaphthalene portion separated bydistillation from recycle oil in the FCC process. The method of thisinvention can also be used to isolate 2,6-dimethylnaphthalene fromdimethylnaphthalene-containing fractions obtained from coal tar. Themethod of this invention can also be used to isolate, bycrystallization, 2,6-dimethylnaphthalene produced via the methoddisclosed in U.S. Pat. Nos. 5,008,479; 5,023,390; and 5,068,480.

The crystallization apparatus most suitable for crystallizing2,6-dimethylnaphthalene according to the method of this invention is ascraped-wall type crystallizer. In these types of crystallizationapparatus spring-loaded scraper blades, typically manufactured from aflexible, polymeric material having resistance to high temperatures,resistance to chemical attack, and good wear resistance and lubricity(for example, nylon, Teflon®, Torlon®, such as Torlon® 4301 or 4302, orHydlar®), rotates within a crystallization drum while the polymericscraper blades "scrape" the inside wall of the crystallization drum. Theoutside of the crystallization drum is equipped with cooling jackets tocool the liquid being crystallized to the desired crystallizationtemperature while the blades in the scraped-wall crystallizer scrapealong the inside cylindered walls of the crystallization drum. Thescraped-wall crystallizer suitably contains a separate pumping device tomaintain the contents of the crystallization drum well mixed. Suitablescraped-wall crystallizers are available from Victoria Machine Works,Victoria, Tex. Two or more scraped-wall crystallizers can be used inseries, each operating at successively decreasing temperatures, andwhere the feed to the second comprises a slurry produced in the firstcrystallizer. A double-pipe scraped-wall crystallizer can also be usedalong with the drum-type scraped-wall crystallizer. While scraped-wallcrystallizers are preferred for the method of this invention, othercrystallization apparatus can be used, for example, a draft tubecrystallizer. However, with these other crystallizers, a lower HM/LMratio will likely be necessary, i.e., where A is no more more than about0.6. The crystallization method disclosed herein can be conducted in abatch or continuous manner.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 is a graph showing the operating range for the method of thisinvention. The region below the line termed "operable," identifies thevarious crystallization temperatures and ratios of HM/LM that can beused at such temperatures to provide for the efficient crystallizationof 2,6-dimethylnaphthalene in a scraped-wall crystallizer. In FIG. 1,the line establishing the operable region corresponds to the equation:##EQU5##

FIG. 2 is a flow diagram representing a preferred embodiment foroperating the method of this invention. In FIG. 2, the dotted linesrepresent optional process steps that can be operated individually or inany combination. In this preferred embodiment, butadiene and o-xyleneare reacted in a liquid phase reaction using a molar excess of o-xylenerelative to butadiene and catalyzed by a metallic sodium-potassium (NaK)catalyst. In the next process step, the excess o-xylene is removed,typically by distillation, and is recycled to the alkenylation reactionmixture. The orthotolylpentene (OTP) separated from the o-xylene iseither sent directly to the cyclization reactor in the Cyclization Stepor subjected to a Separation Step A, typically distillation, to removecomponents having a boiling point lower and/or higher than about theboiling point the OTP, thereby forming an OTP concentrate. The OTP issubsequently cyclized to 1,5-dimethyltetralin (1,5-DMT) in theCyclization Step using a suitable cyclization catalyst, and where thecyclization can be conducted in either liquid or gas phase mode. The1,5-DMT produced from the Cyclization Step is either sent directly tothe Dehydrogenation Step or first subjected to a Separation Step B,typically distillation, where components having a boiling point lowerand/or higher than about the boiling point of 1,5-DMT are removed. Inthe Dehydrogenation Step, the 1,5-DMT is dehydrogenated to1,5-dimethylnaphthalene (1,5-DMN) in either a liquid phase or gas phasereaction using a suitable dehydrogenation catalyst. The product mixturefrom the Dehydrogenation Step is sent directly to the Isomerization Stepor subjected to a Separation Step C, typically distillation, where thematerials having a boiling point lower and/or higher than about theboiling point of 1,5-dimethylnaphthalene are removed. The product1,5-dimethylnaphthalene from the Dehydrogenation Step is isomerized inthe Isomerization Step in either a gas or liquid phase reaction using asuitable isomerization catalyst to form a mixture ofdimethylnaphthalenes, including the desired 2,6-dimethylnaphthalene. Themixture produced by the Isomerization Step is directed to a SeparationStep D, typically distillation, where a portion of the lights and atleast a portion of the heavies can be removed. The resulting moltenmixture containing DMN isomers is directed to the Crystallization Stepwhere the mixture is cooled to crystallize the desired2,6-dimethylnaphthalene in a crystallization apparatus. After thecrystallization, which is preferably conducted in a scraped-wallcrystallizer or two or more scraped-wall crystallizers in series, at atemperature in the range of about 50° to about 150° F., more preferably60° to 140° F., the crystalline 2,6-dimethylnaphthalene is separatedfrom the crystallization mother liquor in Separation Step E using asolid-liquid separation device such a centrifuge, filter, settling tank,etc. The recovered 2,6-dimethylnaphthalene can be subjected toadditional purification procedures, if desired, such as washing,crystallization from a melt of the product at a temperature higher thanthe temperature used in the Crystallization Step, recrystallization froma solvent such as a low molecular weight carboxylic acid such as aceticacid, a low molecular weight alcohol including methanol, ethanol,isopropanol and the like, or a low boiling hydrocarbon such as hexane,octane, nonane, a mixture of low boiling hydrocarbons, or a combinationof such purification procedures. The mother liquor recovered from thecrystallization of the 2,6-dimethylnaphthalene contains lights,2-methylnaphthalene, a mixture of dimethylnaphthalenes including2,6-dimethylnaphthalene that was not crystallized, a mixture oftrimethylnaphthalenes, and various heavy components that have boilingpoints higher than about the boiling points of thetrimethylnaphthalenes. The mother liquor can be separated in SeparationStep F, preferably by fractional distillation, into lights, thedimethylnaphthalenes and the heavies. The lights can be recycled to theCrystallization Step. The dimethylnaphthalenes isomer fraction, as shownin FIG. 2, is preferably recycled to the Isomerization Step. Some of thedimethylnaphthalene fraction may also eliminated after Separation Step Ein order to prevent a build-up of 2,7-dimethylnaphthalene. Some or allof the heavies are removed from the overall process and are disposed asfuel or sold as by-product.

In the method of this invention, the composition of the mixture in theCrystallization Step in FIG. 2, is such that the equation, ##EQU6## issatisfied, where HM, LM and T are described hereinabove, and A is nogreater than 1.0. In the method of this invention, the HM/LM ratio isefficiently adjusted by adjusting the content of the lights in themixture sent to the Crystallization Step. The adjustment in the amountof lights is preferably made during the Separation Step D shown in theFIG. 2 where the product from the Isomerization Step is subjected to aseparation procedure, preferably one or more fractionations, wherelights and heavies are removed from the product formed during theIsomerization Step. In this fractionation step, the amount of lights andheavies included in the material sent to the Crystallization Step can beadjusted to meet the requirements of equation (5). Lights from each ofSeparation Steps A-C can optionally be added to the Crystallization Stepin order to have the composition that is crystallized conform toequation (5) hereinabove. Although not depicted in FIG. 2, 1,5-DMTproduct produced in the Cyclization Step can be added to the mixturecrystallized in the Crystallization Step. As discussed hereinabove,1,5-DMT is a low melting component and can be used to adjust the HM/LMratio so that it meets the requirements of equation (5).

The following examples will serve to further illustrate the method ofthis invention; however, they are not intended to limit the scopethereof.

Feed mixtures corresponding to that shown in the following Tables 1 and2 were crystallized at the temperatures indicated in the tables using ascraped-wall crystallization apparatus as described hereinabove. Thecrystallizer had a volume of 5 gal. and the scraper had Teflon® bladesscraping the crystallizer wall. The feed mixtures used for thecrystallization runs reported in the tables were prepared by thealkenylation process. As shown, the feed mixtures contained variousamounts of lights or 1,5-dimethyltetralin as some of the low meltingcomponents. The feed materials were processed through the scraped-wallcrystallizer in a continuous manner. Samples were analyzed by gaschromotography to determine their composition. Also, the scraped-walledcrystallizer used for these experiments was equipped with a viewing portto allow for the observation of the internals of the crystallizer to seeif there was an unusual or excessive build-up of rime. Eachcrystallization run lasted approximately 16 hours.

As the results in Tables 1 and 2 show, when the HM/LM ratio for thecrystallization feed at crystallization temperature T was less than thevalue (0.03 (T-50)+A), the crystallization was successful in thescraped-wall crystallizer producing 2,6-dimethylnaphthalene in highyield and purity. However, when the value of HM/LM was not less than(0.03 (T-50)+A), the crystallizer was inoperative.

                                      TABLE 1    __________________________________________________________________________    Feed Components Crystallization Run #    (wt. %)         1  2     3  4  5  6    __________________________________________________________________________    "Lights".sup.a  24.0                       24.0  35.3                                7.2                                   0.7                                      4.0    2-MN            8.2                       8.2   12.1                                3.1                                   0.6                                      0.9    1-MN            2.2                       2.2   3.2                                1.1                                   0.5                                      0.3    1,5-DMN         2.1                       2.1   1.6                                7.5                                   13.3                                      9.2    1,6-DMN         6.5                       6.5   5.0                                27.6                                   41.2                                      33.0    2,6-DMN         55.7                       55.7  41.8                                43.5                                   39.0                                      41.4    1,7-DMN         0.5                       0.5   0.2                                1.4                                   1.8                                      0.8    2,7-DMN         0.7                       0.7   0.6                                6.1                                   2.4                                      9.3    Heavies         0.1                       0.1   0.1                                2.5                                   0.4                                      1.0    CONDITIONS    HM/LM.sup.b     2.0                       2.0   1.3                                1.5                                   1.2                                      1.6    Crystallization Temp., °F.                    85 70    70 70 85 70    0.03 (T-50) + 1.0                    2.1                       1.6   1.6                                1.6                                   2.1                                      1.6    RECOVERY OF 2.6-DMN    FROM CRYSTALLIZATION    Yield of 2,6-DMN, wt. %                    77.0                       Inoperable                             66.7                                77.8                                   75.6                                      Inoperable    Purity of 2,6-DMN, wt. %                    97.9                       NA    97.7                                97.5                                   99.0                                      NA    __________________________________________________________________________     .sup.a Lights other than 1MN produced during the preparation of     2,6dimethylnaphthalene by the alkenylation process. Lights have a boiling     point below about 470° F., at atmospheric distillation pressure.     .sup.b Weight ratio of high melting (HM) components, i.e., 1,5DMN, 2,6DMN     2,7DMN and 2MN, in feed mixture to low melting (LM) components, i.e.,     1,6DMN, 1,7DMN, 1MN and other lights, and heavies.

                  TABLE 2    ______________________________________    Feed Components   Crystallization Run #    (wt. %)           7       8      9      10    ______________________________________    1,5-DMT           38.8    45.2   45.2   4.7    "Lights".sup.a    4.1     4.5    4.5    4.6    2-MN              0.2     0.2    0.2    2.0    1-MN              0.1     0.1    0.1    0.9    1,5-DMN           2.5     2.4    2.4    7.6    1,6-DMN           5.5     4.8    4.8    27.3    2,6-DMN           47.7    41.8   41.8   43.0    1,7-DMN           0.3     0.3    0.3    1.2    2,7-DMN           0.6     0.5    0.5    6.2    Heavies           0.1     0.4    0.4    2.6    CONDITIONS    HM/LM.sup.b       1.0     0.8    0.8    1.4    Crystallization Temp., °F.                      70      70     50     70    0.03 (T-50) + 1.0 1.6     1.6    1.0    1.6    RECOVERY OF 2.6-DMN    FROM CRYSTALLIZATION    Yield of 2,6-DMN, wt. %                      67.8    67.5   71.0   76.6    Purity of 2,6-DMN, wt. %                      99.9    99.9   99.9   95.4    ______________________________________     .sup.a Lights other than 1MN and 1,5DMT produced during the preparation o     2,6dimethylnaphthalene by the alkenylation process. Lights have a boiling     point below about 470° F., at atmospheric distillation pressure.     .sup.b Weight ratio of high melting (HM) components, i.e., 1,5DMN, 2,6DMN     2,7DMN, and 2MN in feed mixture to low melting (LM) components, i.e.,     1,6DMN, 1,7DMN, 1MN and 1,5DMT and other lights, and heavies.

While only certain embodiments have been set forth, alternativeembodiments and various modifications will be apparent from the abovedescription to those skilled in the art. These and other alternativesare considered equivalents and within the spirit and scope of thepresent invention.

That which is claimed is:
 1. A method for recovering crystalline2,6-dimethylnaphthalene comprising crystallizing in a scraped-wallcrystallizer apparatus at crystallization temperature T, a mixture oflow melting components, (LM), having melting points of 70° F. and below,and high melting components (HM), including 2,6-dimethylnaphthalene,having melting points above 70° F., such that: ##EQU7## where HM is thetotal weight percent high melting components, including2,6-dimethylnaphthalene, in the mixture, and LM is the total weightpercent of low melting components in the mixture, and where T is thetemperature of the crystallization in degrees Fahrenheit, and where A isno greater than about 1.0.
 2. The method of claim 1 wherein the mixtureis prepared by a process comprising, alkenylating o-xylene with1,3-butadiene in the presence of an alkali metal to formorthotolylpentene, cyclizing orthotolylpentene to form1,5-dimethyltetralin, dehydrogenating the dimethyltetralin to form amixture comprising 1,5-dimethylnaphthalene, and isomerizing the1,5-dimethylnaphthalene to form the mixture comprising2,6-dimethylnaphthalene.
 3. The method of claim 1 wherein the value A isabout 1.0 to about 0.9.
 4. The method of claim 1 wherein the temperatureT is about 50° F. to about 150° F.
 5. The method of claim 4 wherein twocrystallizers are used in series and where the first is operated at ahigher temperature than the second.
 6. The method of claim 2 wherein thehigh melting components, HM, comprise 2,6-dimethylnaphthalene,1,5-dimethylnaphthalene, 2,7-dimethylnaphthalene and 2-methylnaphthaleneand the low melting components, LM, comprise heavies produced during thealkenylation process, lights produced during the alkenylation process,and 1,6- and 1,7-dimethylnaphthalene.
 7. The method of claim 2 whereinthe low melting components include 1,5-dimethyltetralin.
 8. The methodof claim 2 wherein the low melting components comprise lights producedduring the alkenylation process and wherein the mixture crystallized inthe scraped-wall crystallizer contains at least about 5 weight percentlights.
 9. The method of claim 6 wherein the lights comprise componentshaving boiling points less than the boiling point of 2-methylnaphthalenepresent in the mixture, and heavies comprise components having boilingpoints higher than the boiling point of any of the dimethylnaphthalenesin the mixture.
 10. A method for crystallizing 2,6-dimethylnaphthalenecomprising maintaining, at a temperature sufficient to cause thecrystallization of 2,6-dimethylnaphthalene, a mixture comprising2,6-dimethylnaphthalene, at least one of 1,6- or1,7-dimethylnaphthalene, and at least about 5 weight percent lightsproduced during the preparation or isolation of 2,6-dimethylnaphthalenewherein the lights have a boiling point lower than about 470° F.
 11. Themethod of claim 10 wherein the mixture is prepared by a processcomprising, alkenylating o-xylene with 1,3-butadiene in the presence ofan alkali metal to form orthotolylpentene, cyclizing orthotolylpenteneto form 1,5-dimethyltetralin, dehydrogenating the dimethyltetralin toform a mixture comprising 1,5-dimethylnaphthalene, and isomerizing the1,5-dimethylnaphthalene to form the mixture comprising2,6-dimethylnaphthalene.
 12. The method of claim 11 wherein the mixturecomprises at least about 7 weight percent lights.
 13. The method ofclaim 11 wherein the temperature is about 50° F. to about 150° F. 14.The method claim 13 wherein the mixture contains at least about 20weight percent 2,6-dimethylnaphthalene.
 15. The method of claim 11wherein the lights include lights formed during the alkenylating,cyclizing, dehydrogenating and isomerizing steps.
 16. The method ofclaim 10 wherein the lights have a boiling point of less than about theboiling point of 2-methylnaphthalene.
 17. The method of claim 1practiced in a batch manner.
 18. The method of claim 1 practiced in acontinuous manner.